fix: update brand token

Author: QuintonJason

packages/sage-assets/lib/stylesheets/core/_variables.scss

@@ -15,7 +15,7 @@ $sage-greet-font-path: "#{$sage-font-cdn-root}/greet";
 ///
 $sage-active-underline-configs: (
   height: rem(3px),
-  color: var(--pine-color-mercury-500),
+  color: var(--pine-color-brand),
 );
 
 ///